Severe bleaching events are hitting coral reefs five times as often as in 1980, researchers report in the Jan. 5 Science.

Scientists surveyed 100 coral reef locations in tropical zones around the world, tracking each spot’s fate from 1980 to 2016. At first, only a few of the locations had experienced bleaching. But by 2016, all had been hit by at least one bleaching event, and all but six had suffered a severe event — defined as affecting more than 30 percent of corals in an area.

The median time between pairs of severe bleaching events has also decreased, the researchers found — it’s now just under six years, versus 25 to 30 years in the early 1980s. That’s not enough time for corals to fully bounce back before getting hit again.

Consistently higher tropical water temperatures, the result of climate change, are in part to blame for the increase in bleaching, researchers say. Warm water stresses corals and strips away their symbiotic algae — their main source of food and the reason they’re colorful. Bleaching episodes can be fatal, especially if corals can’t recover between events.

In the past, major bleaching events were most likely to happen when El Niño brought bands of warmer water to the tropics. But sea surface temperatures in tropical areas are now warmer during today’s La Niña years (when the water is typically cooler) than during El Niño events 40 years ago, says study coauthor Terry Hughes, a coral researcher at James Cook University in Townsville, Australia. Because those temperatures continue to rise, “we have a narrowing window of opportunity to save reefs,” he says.

Takeover

In 1980, most of the coral reefs tracked in a new study hadn’t been bleached. Now, all the sites have had at least moderate bleaching, and almost all have experienced at least one severe event (blue lines). Researchers recorded more than 600 total bleaching incidents and about 300 severe events over the 36-year study period (red lines).